In the last issue, we looked at the SA-1 (Ship’s Aeroplane) developed by the U.S. Naval Aircraft Factory at the end of World War I. Although an oldie with many features that were outdated even for 1919, it deserves more attention, as it has many features that are applicable to today’s homebuilts, both in the Ultralight and Experimental Categories.
The basic design could be called “Son of Demoiselle” because it drew so heavily on that unique model. While the “Demoiselle” was a true pioneer in 1908 because it had nothing to copy, the SA-1 is an interesting example of working down from established “Big Airplane” practice to lightweight super-simple bare-minimum flying by the standards of its day.
The purpose of the SA-1 was unique – to fly off a platform built over the turret guns of battleships to scout for the ship — but not land back aboard. With the low speed of the plane, its limited range and the possible necessity of ditching in the water, it is hard to see the practicality of the SA-1 mission concept. With no radio, the “Eye In The Sky” had no way of reporting its information back to the ship. Maybe the pilot was supposed to drop a message.
The Navy did fly some standard single-seat scouts and even some two-seaters from the turrets for a couple of years, however, before the catapults for ship-based seaplanes were perfected. Again, however, no radio.
The simplicity of the SA-1 was carried almost to the point of crudity, and some abandoned obsolete features were readopted for it. Still, the design has much to offer present-day homebuilders, and is worth studying from that point of view.
The most notable feature, of course, was the open fuselage. This had pretty well died out for tractor designs well before World War I, but is pretty much standard for today’s ultralights. The powerplant up by the leading edge of the wing is straight “Demoiselle” and is frequently seen today.
Some of the moderns have the propeller on the front of the engine as a tractor while others like “Quicksilver” still carry the engine up front but put a pusher prop behind the wing on an extension shaft.
Two major obsolete features readopted for the SA-1 were the pitch and roll controls. There was no fixed horizontal stabilizer, just elevators. This detail was a commonly used weight-saving feature well into World War I, but died out until resurrected for gliders in the 1920s and 1930s. Called “Pendulum” elevators, they had the disadvantage of no “feel” and it was very easy to overcontrol with them. The Bowlus “Baby Albatross” kitbuilt sailplane of 1939 had them, but the FAA wouldn’t license the Baby until springs were put in the system to provide some artificial feel. While this was only in proportion to surface displacement, not actual air load, it was better than nothing. In Germany in the 1930s, gliders with pendulum elevators were prohibited from cloud flying.
Elevators without fixed stabilizers have reappeared on light aircraft in recent years (thanks to John Thorp) as the “Slab Tail” or “Flying Tail,” but actual feel is now imparted by automatic resistance tabs in the trailing edge. Homebuilts would do well to stick to stabilizers; they have other advantages, too, such as providing convenient push and lift points when ground handling the plane.
The wings of the SA-1 did not have ailerons, but resorted to the old trick of warping the wings through cables for roll control. This would be quite acceptable on an ultralight today, especially types with the rear spar at the trailing edge of the wing as on “Quicksilver.”
The designers of the SA-1 didn’t realize what a handicap they were building into it with that large gap in the center section. In addition to creating four drag-producing wingtips instead of two, the gap also put a big dent in the spanwise lift distribution curve.
With a span of 27 feet 8 inches and an area of 144 square feet, the SA-1 wing had an aspect ratio of only 5.2. Efficiency could be greatly improved by increasing the span and decreasing the chord for the same area, and closing the center section gap. A lot of weight can be saved by revising the structure along modern ultralight lines and by using modern two-stroke cycle engines. The empty weight of the SA-1 with the 56 hp Lawrance L-3 engine was 425 pounds. Weight of the 223 cubic inch engine was 175 pounds and it delivered its power though a large propeller at 1,600 rpm. The wing structure weighed 120 pounds. At a gross weight of 695 pounds, wing loading was 4.8 pounds per square foot – lots of room for improvement here.
DEMOISELLE DERIVATIVE: The Naval Aircraft Factory SA-1 of late 1918 drew heavily on the layout of the Santos-Dumont “Demoiselle” of 1908. Powerplant was the 55 hp Lawrance L-3 three cylinder radial.
ULTRALIGHT UTILITY: The basic three-longeron fuselage and top mounted engine of both the 1908 and 1918 designs lend themselves to modern ultralight practice. The purpose of the extra “tailskids” just behind the pilot’s seat is to allow the actual tail to project beyond the rear edge of the battleship turret to permit a slightly longer takeoff run.
WIRED: Bracing for the SA-1 wings was by fixed wires from the lower longerons and the top of the forward pylon to the front spars. The warping wires for the rear spars rolled over pulleys at the top and bottom of the rear pylon and also carried the rear spar flight loads.
A GAP IN LOGIC? The gap in the middle of the wing was a huge aerodynamic handicap that was not fully understood in 1918. The wing had in effect four tips, with their inherent drag problems, and the gap was very detrimental to the spanwise lift distribution.
NO ‘FEEL’: A low-aspect-ratio rudder was installed behind a conventional fixed vertical fin, but there was no horizontal stabilizer. The elevators had large balance areas ahead of the hinge line to cut down on stick forces, but these also killed off “feel.”